JP2742619B2 - Silicon nitride sintered body - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a silicon nitride sintered body which is suitable for heat engines such as gas turbines and turbo rotors and has excellent bending strength and toughness at high temperatures.

(Prior Art) Conventionally, silicon nitride-based sintered bodies have attracted attention as engineering ceramics, particularly as materials for heat engines, because of their excellent strength, hardness, and thermochemical stability at high temperatures.

Generally, in order to manufacture these silicon nitride sintered bodies, various sintering aids such as rare earth element oxides are added because silicon nitride itself is difficult to sinter. Or a gas pressure firing method. Also,
Recently, a high-density material has been formed by forming an opaque seal made of glass or the like on the surface of a silicon nitride molded body having a desired composition and firing it under high pressure (hereinafter referred to as seal HIP).
A method for obtaining a high-strength sintered body has been proposed.

On the other hand, from the viewpoint of composition, in addition to rare earth element oxides such as Y ₂ O ₃ as described above, oxides such as Al ₂ O ₃ and MgO are most commonly used as sintering aids. Considering the high-temperature characteristics of the sintered body, if Al ₂ O ₃ or MgO is contained in the sintered body, a low-melting substance is generated at the grain boundaries of the sintered body, resulting in high-temperature strength and high-temperature oxidation resistance. Si ₃ N ₄ -RE ₂ O ₃ (rare earth oxide) which does not substantially contain the above oxides
Simple ternary composition system of -SiO ₂ has been studied.

From the viewpoint of the structure of the sintered body, the grain boundary phase in the sintered body has been attracting attention as a factor that determines the strength and high-temperature characteristics, and the grain boundary phase is intended to improve the strength of the grain boundary phase itself. Attempts have been made to substantially crystallize the phase. Therefore, recently, for the above simple ternary composition, the sintering conditions were examined or the heat treatment of the sintered body resulted in the formation of Si at the grain boundaries.
₃ N ₄ -RE ₂ O ₃ (rare earth oxide) consisting -SiO ₂ various crystal phases, for example apatite, YAM, has also been possible to precipitate wollastonite like.

However, although crystallization of the grain boundary phase has a certain effect on the high-temperature strength, it is very difficult to stably precipitate only a specific crystal phase at the grain boundary. In some cases, a low melting point glass phase other than the crystalline phase is formed, thus deteriorating the characteristics.

Therefore, the present inventors have considered that the above ternary composition
A composition having an SiO ₂ / RE ₂ O ₃ molar ratio of more than ₂ and an excess of SiO _{2 of} 25 or less is fired at a low temperature by the seal HIP method to form a fine nitrided structure, and the grain boundary phase is silicon,
Room temperature strength composed of rare earth elements, oxygen and nitrogen,
It has been proposed that a sintered body excellent in high-temperature strength and oxidation resistance can be obtained.

(Problems to be Solved by the Invention) However, although the above-mentioned sintered body has superior characteristics in bending strength and oxidation resistance as compared with conventional products, variations in characteristics have not been solved yet, and However, it has a disadvantage that it is difficult to obtain a sintered body having stable properties in terms of properties.

(Object of the Invention) Accordingly, an object of the present invention is to provide a silicon nitride-based sintered body having the above-mentioned excellent strength and oxidation resistance and excellent in mass productivity.

(Means for Solving the Problems) The present inventors have studied the above-mentioned Si ₃ N ₄ -RE ₂ O ₃ (rare earth oxide)-
Observation of the structure of the test piece which showed low strength as to the cause of the variation in the SiO ₂ -based sintered body and the source of the fracture showed that abnormal grain growth was present in almost all of the structure. Further, when the structure of the abnormal grain growth portion was analyzed, it was found that iron (Fe) was present, and a low-melting substance was generated by the reaction with the sintering aid, which caused abnormal growth of silicon nitride. I found it.

Therefore, the present inventors have found that abnormal grain growth hardly occurs by minimizing the mixing of iron (Fe) in the raw material or the production process, and the variation in characteristics is eliminated, and the present invention has been accomplished.

That is, the present invention relates to a method for producing silicon nitride of 70 to 99 mol%, rare earth element oxide of 0.1 to 5 mol%, and excess oxygen of 25 mol% or less in terms of SiO ₂ (excess oxygen / rare earth element oxide). The iron (Fe) content in the silicon nitride sintered body having a fine structure in which the molar ratio is larger than 2 and is 25 or less and the average crystal grain size of the silicon nitride crystal is 7 μm or less is 30 ppm.
The object is achieved by reducing the following.

 Hereinafter, the present invention will be described in detail.

A major feature of the present invention is that the composition is silicon nitride 70
~ 99 mol%, especially 80-93.5 mol%, rare earth element oxide
0.1 to 5 mol%, especially 0.5 to 4 mol%, excess oxygen (SiO ₂ conversion) is 25 mol% or less, especially 6 to 20 mol%, and the molar ratio of (excess oxygen / rare earth element oxide) is 2
Larger, less than or equal to 25, especially 3 to 20. The excess oxygen is the amount of oxygen obtained by removing the stoichiometric amount of oxygen mixed as a rare earth element oxide from the total amount of oxygen contained in the entire sintered body system. Impurity oxygen or oxygen added as SiO ₂ , and in the present invention, any of SiO ₂
It shows the conversion amount.

The present invention focuses on the fact that iron (Fe) as an impurity has a great effect on its properties when sintering at a relatively low temperature using the above composition to obtain a sintered body having a fine structure structure. It is.

Although the reason is not clear, the present inventors consider as follows.

When sintering is performed at a high temperature by, for example, gas pressure sintering or the like using this composition system, sintering proceeds due to the generation of a low-viscosity liquid phase by the auxiliary component. It is considered that the impurity metal has almost no influence on the characteristics because the impurity metal is dispersed in the grain boundary having a low viscosity. However, when the above composition is fired at a low temperature by a method such as hot isostatic firing, a liquid phase is generated by an auxiliary component in the sintering process, but the sintering is carried out while the viscosity is kept very high. proceed. At this time, if an impurity metal such as iron is present in the grain boundary, a low melting point substance is generated by a reaction with an auxiliary agent and the like, and a low melting point generated due to a high viscosity of the other grain boundary itself. It is presumed that only the substance moves in the grain boundary and agglomerates, and silicon nitride grows abnormally in the agglomerated portion and becomes a destructive source.

The present invention has been studied on the amount of iron (Fe) based on the above findings, and is characterized in that the amount is controlled to 30 ppm or less in the sintered body, thereby affecting the characteristics as described above. And the variation in the characteristics of the sintered body can be reduced.

As a method for producing a silicon nitride-based sintered body having the above-described configuration, it is basically necessary to bake the above-described composition at a low temperature, and it is necessary to minimize the mixing of iron. As a specific example, the seal HIP method is suitable. After that, seal HIP
The method will be described as an example.

First, a silicon nitride powder, a rare earth element oxide powder, and optionally a SiO ₂ powder are used as raw material powders. Silicon nitride powder has a BET specific surface area of 3 to ²⁰ m ² /
g, desirably at least 95%. The oxygen content is generally but contained about 0.8 to 1.4% by weight commercially available, it can be arbitrarily adjusted by the addition of SiO _2.

First, most of the iron mixed into the system is an impurity in the raw material powder. Normally, iron is present as 60 ppm or more as a metal impurity in the silicon nitride powder. According to the present invention, it is most efficient to first remove the iron in the silicon nitride powder.

As a method for removing iron in the raw material, for example, the raw material can be easily removed by washing with an acid such as hydrochloric acid or nitric acid.

Next, the powder thus treated is weighed and mixed with the composition described above, a binder is added, and the mixture is granulated and then molded. The molding can be performed by a known method, and specifically, press molding, extrusion molding, casting molding, injection molding, or the like can be employed.

After debinding the molded body, a powder having poor wettability with glass, such as BN powder, is applied to the surface of the molded body for the purpose of preventing a reaction with glass or the like as a sealing material in a firing step. The powder such as BN or the like having poor wettability with glass may be applied to the surface of the molded body by slurrying the powder of BN or the like and applying the slurry to the molded body, or by spraying the slurry. The thickness of the coating on the surface of the molded product is 1 to
About 10 mm is desirable.

A drying step is required after powder application of the above BN etc., and cracks are likely to occur in the molded body at this time. Therefore, the molded body before BN application should be calcined once in an inert gas atmosphere at 1200 to 1600 ° C. Is desirable.

In addition, impurities such as B ₂ O ₃ are present in the BN powder and are mixed into the sintered body during sintering to form low melting point grain boundaries and deteriorate the high temperature characteristics of the sintered body. Before firing, 1200 ~
It is desirable to perform a heat treatment under a reduced pressure of 1450 ° C. to remove the impurities. Note that it is necessary to perform the conditions at this time under conditions that do not cause decomposition of silicon nitride.

Next, a glass powder that forms a seal at the time of firing is applied to the surface of the molded body coated with BN, or is encapsulated in a glass capsule. As another method, the molded body can be buried in a heat-resistant container filled with glass powder. Thereafter, the molded body is fired under high temperature and high pressure by the HIP method.

In the firing by the HIP method, after removing the water contained in the molded body by raising the temperature in the furnace under reduced pressure, at the same time as raising the temperature to the firing temperature equal to or higher than the softening point of the glass present on the surface of the molded body, The glass is softened while introducing a nitrogen gas having a partial pressure equivalent to or higher than the decomposition equilibrium pressure of silicon nitride at the temperature by about 0.01 to 0.2 MPa to form a gas impermeable film made of glass on the surface of the molded body. After the gas-impermeable membrane is completely formed on the surface of the molded body, the pressure is raised to a pressure of, for example, 50 MPa or more under conditions where the pressure in the furnace can be sufficiently densified using an inert gas such as nitrogen or argon as a pressure medium. Let it. At this stage, a liquid phase is generated by the rare earth oxide, SiO ₂ and silicon nitride, and the firing proceeds, and the densification is almost completed. Thereafter, the crystal is grown while maintaining the maximum temperature and pressure for a predetermined temperature period, and then the temperature and pressure are reduced to complete the firing.
The maximum temperature at this time is 1450-18
The temperature is preferably in the range of 00 ° C, but in particular, 1500 to 1750 ° C is desirable for miniaturizing silicon nitride crystal to 7 µm or less.

In the sintered body thus obtained, the α-silicon nitride crystal phase remains together with the fine β-silicon nitride crystal phase,
These crystal phases consist of fine particles having an average particle size of 7 μm or less, and silicon, rare earth elements, oxygen and nitrogen are present at the grain boundaries of the crystal particles. According to the above manufacturing method, a silicon oxynitride crystal phase represented by Si ₂ N ₂ O or a disilicate represented by RE ₂ O ₃ .2SiO ₂ (RE: rare earth element) in addition to a high melting point glass phase A crystalline phase may form. Such a grain boundary structure has advantages in that it has excellent properties in high-temperature strength and oxidation resistance as compared with conventionally known various crystal phases and can be manufactured stably.

In addition, abnormal grain growth of silicon nitride is suppressed due to the reduced iron content, and a sintered body having excellent property stability can be obtained.

In addition, the characteristics can be improved by subjecting this sintered body to heat treatment under a predetermined condition, for example, at a temperature of 1200 to 1700 ° C. in a non-oxidizing atmosphere, or heat treatment in an oxidizing atmosphere.

In the present invention, the composition of the sintered body is limited to the above-described ratio, which is an important factor for obtaining excellent characteristics, and any one of silicon nitride, rare earth oxide, and excess oxygen deviates from the above-described range. However, the room-temperature strength and the high-temperature strength are reduced, and when the molar ratio (excess oxygen / rare earth element oxide) is 2 or less, the oxidation resistance at high temperatures is liable to be deteriorated. This is because high-temperature characteristics are easily deteriorated. Further, the average crystal grain size is set to 7 μm or less, because the room temperature strength and the high temperature strength are increased, and the grain growth is promoted. If the grain size exceeds 7 μm, the desired strength cannot be obtained.

The rare earth element used in the present invention is Y
Is most common, but heavy rare earth elements such as Er, Yb, Ho, Dy, and Gd are desirable in view of stability of characteristics.

 Hereinafter, the present invention will be described with reference to the following examples.

(Example) Silicon nitride powder (BET specific surface area 5 m ² /
g, α conversion 99%, impurity oxygen content 1.0 wt%, Fe content 62pp
m) and various rare earth oxides or SiO ₂ powders.
Note that the silicon nitride powder was subjected to an acid cleaning treatment with hydrochloric acid to reduce the iron content.

Using these powders, they were blended to have the composition shown in Table 1, mixed, press-molded at 1 t / cm ² , and then calcined at 1400 ° C.

After applying a paste of BN powder having a particle size of 1 to 5 μm to a thickness of 1 to 10 mm on the obtained molded body, heat treatment was performed at 1350 ° C. under a reduced pressure of 0.2 Torr to remove impurities.

Thereafter, a glass containing SiO ₂ as a main component was applied in a thickness of 1 to 10 mm.

The green body thus treated was placed in a hot isostatic firing furnace and fired under the firing conditions shown in Table 1.

Next, the iron content of the sintered body after removing the glass was
In addition to quantification by P analysis, 14 test pieces were cut out from each sintered body, and the average values of the four-point bending strength at room temperature, 1200 ° C and 1400 ° C and the increase in oxidized weight at 1400 ° C were determined in accordance with JISR1601. Was.

In addition, the fracture sources of eight bending test specimens were observed for each sample, and the number of fracture sources whose abnormal grain growth was caused by iron was examined.

The average particle size of the silicon nitride crystal particles was calculated from an electron micrograph of the sintered body.

 The results are shown in Table 1.

Further, as a comparative example, a molded body was produced using two kinds of raw materials having different amounts of iron, fired by a gas pressure firing method under the conditions shown in Table 1, and the characteristics were similarly evaluated.

FIG. 1 shows an electron micrograph of the microstructure of the abnormal grain growth portion, which was the fracture source of the bending test piece, for the sample No. 4 in Table 1.

According to Table 1, Sample No. whose iron content exceeds 30 ppm
In cases 3 and 4, the silicon nitride crystal grain size was 7 μm due to low-temperature firing.
As many as 2/8 or more of the following microstructures and fracture sources caused by abnormal grain growth due to iron, this is a major cause of variation, and as a result, compared to other samples of the same component and composition As a result, the strength deteriorated. In addition, from the center of the fracture source shown in FIG.
Was detected, and it was understood that the surrounding area was abnormally grown.

In contrast, the product of the present invention in which the amount of Fe was suppressed to 30 ppm or less
The number of fracture sources due to abnormal grain growth due to e was as low as 1/8 or less, most of them showed grain boundary fracture, there was almost no abnormal grain growth, and the properties were good.

On the other hand, in terms of composition (excess oxygen / rare earth element oxide)
The sample of No. 7 having a molar ratio of less than 2 has poor oxidation resistance, and the sample of No. 10 having a molar ratio of more than 25 has a low strength at 1400 ° C.

In addition, as a comparative example, in the samples of Nos. 17 and 18 in which the composition within the scope of the present invention was fired at a high temperature under nitrogen gas pressure, there was no abnormal grain growth due to iron on the structure, and the fracture source was grain boundary fracture. It was found that there was almost no influence of iron. However, both had a large crystal grain size, and were inferior in characteristics to the product of the present invention.

(Effects of the Invention) As described in detail above, according to the silicon nitride sintered body of the present invention, a simple ternary composition of SiO ₃ N ₄ —RE ₂ O ₃ (rare earth oxide) —SiO ₂ (excess oxygen) In a system containing a large amount of excess oxygen, by reducing the iron content in the sintered body, it is possible to reduce the variation in characteristics of the sintered body having excellent strength at room temperature and high temperature.

Accordingly, it is possible to further promote mass production of various mechanical structural parts, such as a high-temperature structural material such as a heat engine of a silicon nitride sintered body, and to enhance the reliability of its characteristics.

[Brief description of the drawings]

FIG. 1 is an electron micrograph showing the microstructure of the abnormal grain growth part due to iron.

Claims (1)

(57) [Claims] 1. 70 to 99 mol% of silicon nitride, 0.1 to 5 mol% of rare earth oxide and 25 mol% of excess oxygen (in terms of SiO ₂ )
A silicon nitride-based sintered body having a molar ratio (excess oxygen / rare earth element oxide) of more than 2 and not more than 25, and having an average particle diameter of silicon nitride crystal of 7 μm or less, A silicon nitride-based sintered body characterized in that the content of iron (Fe) in the sintered body is 30 ppm or less.